Ink printing apparatuses can be used for single or multicolor printing of a printing substrate web, for example a belt shaped recording medium made of the most varied materials (for example paper). The design of such ink printing apparatuses is known, see for example EPO 788882 B1. Ink printing apparatuses that operate according to the drop-on-demand (DoD) principle have a print head or multiple print heads with nozzles comprising ink channels, the activators of which nozzles—controlled by a printer controller—excite ink droplets in the direction of the printing substrate web, which ink droplets are deflected onto the printing substrate web in order to apply printing dots there for a print image. The activators can generate ink droplets thermally (bubble jet) or piezoelectrically.
In the printing of a printing substrate web it is sometimes necessary to halt the printing substrate web during the printing operation in a pause function, for example in order to monitor the register quality after printing a print job, or to correct problems in the post-processing of the printing substrate web. After running up the printing substrate web again, print image disruptions can then occur at those web segments that were located directly under the printing nozzle after activation of the pause function. Due to the relatively large transfer printing zone in ink printing apparatuses (for example inkjet printing systems), in particular in color printing, the print image disruptions that occur due to the pause cause a correspondingly large amount of spoilage. The occurring print image defects include print image distortions or color register errors. The causes for these are the swelling or shrinking of the printing substrate web during the pause and, connected with this, the position shifts of the printing substrate web below the printing heads.
These problems are explained using FIG. 1. A printing unit 1 and a printer controller 2 of a printing apparatus DR are shown. Arranged along a printing substrate web 3 is a printing unit 1 that has print bars 4 with print heads 5 one after another as viewed in the transport direction of the printing substrate web 3. In color printing, for example, a respective print bar 4 can be provided per color to be printed. The printing substrate web 3 is moved past the print bars 4 with the aid of a take-off roller 9. The printing substrate web 3 is thus situated on a saddle with guide rollers 8. Arranged at the input of the printing unit 1 is a rotary encoder roller 6 that is driven by the printing substrate web 3 and that generates rotary encoding pulses depending on the feed movement of the printing substrate web 3, which pulses are supplied to the printer controller 2 and are used by the printer controller 2 in order to establish the point in time of the triggering of the printing process at the individual print heads 5. The printing substrate web 3 is supplied to the rotary encoder roller 6 via a drive roller 7 arranged before the rotary encoder roller 6.
In FIG. 1 it is shown in principle how the printing substrate web 3 can be affected by the printing unit 1 or the environment air in individual web segments BA through the printing apparatus DR, for example given a downtime of the printing apparatus DR. In the web segment BA1 between drive roller 7 and rotary encoder roller 6, the printing substrate web 3 is exposed to the environment air, with the result that here a swelling of the printing substrate web 3 can occur due to the humidity of the environment air. However, the change to the printing substrate web 3 in the longitudinal direction that is caused by this is compensated with the aid of the rotary encoder roller 6. In the web segment BA2 after the rotary encoder roller 6 and up to the printing unit 1, a swelling of the printing substrate web 3 can occur due to the environment air, this swelling is not taken into account by the rotary encoder roller 6, however. This also applies to the web segment BA3 below the print heads 5 of the printing unit 1. There the printing substrate web 3 can shrink due to the operating temperature of the print heads 5; however, the printing substrate web 3 is also exposed to the environment air, such that—in particular given larger distances between the print bars 5—the web segment BA3 can swell due to the moisture in the environment air. Both influences overlap. The printing substrate web 3 is thus exposed to different environmental influences from the driver roller 7 up to the take-off roller 9, which can lead to either a shrinking or a swelling of the printing substrate web 3. This can lead to the aforementioned print image errors, in particular if the printing process is restarted after a pause during the printing operation.
Upon triggering the pause function, the following effects on the printing substrate web 3 are thus to be considered:                a temperature and moisture difference between the printing substrate web 3 and the environment air, with the swelling or shrinking of the printing substrate web 3 that is concurrent with this; and        a temperature difference between the print heads 5 and the printing substrate web 3, with the shrinking of the printing substrate web 3 that is concurrent with this.        